It is well known that the dry sorting apparatus used in industrial production at home and abroad are mainly pneumatic jig and air table. The method of using air to replace water as sorting medium has already been eliminated through selection gradually for its poor sorting effect, low production capacity and a large amount of wind power demand.
An adverse current sorting apparatus was disclosed by American Graham & Troman Co., Ltd in Volume one of the book Coal-beneficiation Technique published in 1988. It's a kind of air table, whose bed surface is rectangular and is provided with an inclination of 10°. The longitudinal vibration device makes the bed surface move longitudinally and the bed surface adopts a perforated plate. A blower fan sends wind under the plate at a constant wind speed and pressure. The upward flowing airflow makes the materials on the bed surface sorted in layers and then the heavier waste-rock falls onto the perforated plate through the table and move upward and slantways along the bed surface effected by eccentric to-and-fro movement. In this way, the lighter coal emerges at the upper layer on the table and is pushed by the fed-in materials to move along the downwards-slanting bed surface. In nature, it's a kind of air-fluidized bed. The sorting is completed on the bed surface at a time. The float coal is discharged at a time, but it is easy for fine grain waste-rocks to drop in the coal. Therefore, the sorting effect is very poor, the production rate is low and the demanded wind power is very large. There is also another kind of dry type shaking table, which needs no wind power and depends on the helical motion of the materials to make the materials sorted in layers. In this way, the coarse grains stay at the upper layer and the fine grains stay at the lower layer. So, the sorting effect is poor because it is made according to the density of the materials.
A combined type sorting method was published in 1990s, which adopts the self-born medium (i.e. the fine grain coal contained in the raw coal to be sorted) and air to form air-solid two-phase medium for sorting; which utilizes mechanical vibration to sort the materials time after time by making them roll spirally; and which makes the most of the buoyancy effect of the interaction between grains generated by the gradually increased density of the materials on the table to sort the materials. Under such a condition that the materials are made to roll spirally on the bed surface of the sorting apparatus, the combined action of the vibratory force and the ascending air is utilized to make the material layers on the table incompact and make the mineral grains layered according to density. Under the effect of gravity, the materials whose density on the surface layer is low are delaminated. While this action is repeated, the materials are sorted time after time. By delaminating layer upon layer, several kinds of products are produced whose ash contents are from low to high, until the waster rocks and iron pyrites are discharged. Then, sorting is made by making use of the fine grain materials in the fed-in raw coal (called self-born medium) and upward flowing gas to form air-solid two-phase medium. In this dispersion medium with a certain density, the low-density materials emerge upward and the high-density materials sink. Then, pure waste-rock product is obtained by making use of the buoyancy effect generated by the interaction of the heavier grains to make the coal grains mixed in the waste-rock layer emerge continuously.
CN2101532 and CN2314850 respectively disclosed the above combined type of dry sorting apparatus, which are applicable to sorting material of solid particle admixture, in which the density of the solid particles is different. The sorting apparatus consist of a vibrator and a sorting bed hung from the frame to form a vibration bed. The vibration bed is provided with lattice bars according with the direction of the motion of materials. The bed surface between lattice bars is equably provided with vertical wind ports connected with the air chambers. A slanting backboard is attached to the back of the sorting bed surface. Affected by the vibratory force and upward flowing gases, the materials move spirally and are sorted into layers according to density. Through circulatory sorting time after time, the low-density materials are separated from the high-density materials.
However, the following problems exist in the sorting beds disclosed by above two patents:
Firstly, the above patents didn't disclose the obliquity of the bed surface of which the lateral angle and longitudinal angle directly affect the sorting effect. If the lateral angle is too small, the thickness of the materials on the table will increase and the sorting speed of the coal on the surface will decrease. If the lateral angle is too large, the vibration speed of the high-density materials at the bottom will decrease, or may not move at all, which will ruin the sorting process. If the longitudinal positive angle (waste-rock discharging end is lower than material feeding end) is too large, the waste-rocks and coal will gather into the waste-rock discharging port. If the longitudinal negative angle (waste-rock discharging end is higher than material feeding end) is too large, the waste-rocks will move at an excessively low speed and accumulate too much on the table to ruin the sorting process. Therefore, it is extremely important for the bed surface have a rational obliquity. Secondly, the functions of the parallel lattice bars on the bed surface include: (i) guiding the high-density mineral grains to move to the waste-rock channel; (ii) increasing the speed of the high-density mineral grains at the bottom of materials on the bed surface to move to the waste-rock end, which is propitious to the increasing of the processing rate of the sorting bed; (iii) sorting function to a certain extent. The low-density coal at the top of the materials on the bed surface can cross over the lattice bars to move to the material discharging side and the high-density materials at the bottom of the materials on the bed surface will be the held up by the lattice bars and can only move along the lattice bars to the waste-rock end, which increases the sorting accuracy. Therefore, an appropriate height and obliquity of the lattice bars is very important to the effect of sorting. Thirdly, the shape of the bed surface is square pentagon and the proportion of the width of the feeding side to the length of the bed surface is 4:10, resulting in shorter sorting time, easier for waste-rocks to drop in the float coal and more difficult for sorting accuracy to be increased. Fourthly, the above patents didn't provide the degree of the obliquity between the backboard and the bed surface, but the angle of the backboard guides the materials to move spirally, which is very important for the separation of light materials. Fifthly, vibration form of the bed surface is electromagnetic vibration or low amplitude inertial force vibration, which isn't propitious to the sorting of the materials whose partial sizes are comparatively large and which moves the materials at the bottom of the materials on the bed surface at a low speed and it is very difficult to increase the production capacity.